Microstructure evolution and kinetics analysis during tempering of Mn–Ni–Mo nuclear power forging steel

Abstract

The mechanical properties of Mn-Mo-Ni nuclear power forging steel can be affected greatly by tempering processes. In the present paper, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and hardness measurements were employed to correlate property variation with the evolution of microstructure during tempering of an Mn-Mo-Ni steel. Kinetics analysis based on hardness measurements was applied to isothermal tempering at temperatures in the range 600-660uC. The hardness variations during tempering are explained by decomposition of retained austenite and martensite-austenite (M-A), precipitation and coarsening of carbides within bainitic ferrite, and recrystallisation of bainitic ferrite. According to the kinetics analysis, the tempering transformation activation energy has been calculated as about 101?4 kJ mol 21 , which indicates a mechanism of carbon diffusion within ferrite. Finally, the considerable precipitation and coarsening of carbides within bainitic ferrite have been observed only at high isothermal temperatures (generally equal to or above 660uC), indicating a high tempering resistance of the Mn-Mo-Ni steel.